Method and apparatus for preventing pin fraud on coin telephones that use battery reversal pulses to meter charges

ABSTRACT

Pin fraud of battery reversal pulse metered sophisticated coin telephones is detected and prevented by the method and apparatus of the invention. By carefully monitoring within the switch office the loop current that is flowing in the 48 volt battery circuit, especially in the more positive terminal portion of the 48 volt battery circuit, loss of current to a fraud ground at a remote coin telephone can be detected. Once a fraud ground is detected, the call is terminated. The current detector of this method and apparatus can also be used to terminate a call for the normal on-hook break of the loop current. A convenient location in the switch office and an inexpensive component configuration make the addition of this pin fraud detection and prevention apparatus very cost effective.

TECHNICAL FIELD

The invention relates to an apparatus for preventing coin telephone fraud and more particularly to an apparatus for preventing pin fraud on a coin telephone which uses battery reversal pulses to meter the charges at the coin telephone. Such systems are found in Taiwan, R.O.C. and various other locations of the world.

DESCRIPTION OF THE PRIOR ART

Coin telephones, some varieties of which are also known as pay telephones, are usually connected to a local telecommunication switch. The local switch routes the call (at least in part) to the called party and oversees the charging of the call originator for the call. If the called party is sufficiently removed from the coin telephone, timed charges are typically made. The acceptance of payment at the coin telephone is controlled from the local switch by control signals and/or control signal sequences. Certain illegal actions may be taken by an unscrupulous caller to hide, block or mimic the control signals from the local switch in order to defraud the local telephone service provider and obtain free or reduced cost telephone calls from coin telephones. Because in one well known technique a pin is used to pierce part of the handset or the handset cord in order to ground either the tip conductor or the ring conductor, this type of fraud is commonly known as pin fraud.

In the United States, collection of coins for a call is made by operation of a fairly substantial voltage pulse that actuates a mechanical device at the coin telephone. Pin fraud can interrupt the normal coin collection and cause deposited coins to be returned to the fraud perpetrator after the pin fraud device is removed and an incomplete local call attempted. This type of pin fraud is described in U.S. Pat. Nos. 5, 022,073 issued Jun. 4, 1991 to Jordan; 4,989,239 issued Jan. 9, 1991 to McGarry; and 4,862,494 issued Aug. 29, 1989 to Matheny. The other part of pin fraud, which includes fraudulently inducing a dial tone and a free local call, is not applicable to modern "dial tone first" coin telephones.

Some countries of the world, such as Taiwan, use a different, more sophisticated type of coin telephone. They are more sophisticated in that these coin telephones provide for their own coin collection. The local switch thus only must provide audio path plus time metering current pulses that tell the sophisticated coin telephone when a unit charge to the calling party has been incurred. These metering current pulses are sent along the same two wires that carry the audio signals. Pin fraud for these sophisticated coin telephones entails the use a pin to ground the tip lead or conductor at the coin telephone. This grounding shunts enough of each metering pulse to prevent the self-operating coin collection circuitry of the sophisticated coin telephone from detecting its occurrence. Thus, the charging circuitry is defrauded into never charging more than the initial fee, which means fees for long calls and/or long distance calls go uncollected. This type of pin fraud costs telephone companies in Taiwan, R.O.C. millions of dollars per year. Unfortunately, these types of systems are sufficiently different from those used in the United States that the anti-pin fraud circuits used there have been unable to prevent pin fraud in Taiwan, R.O.C. and service areas having similar sophisticated coin telephones.

SOLUTION

The aforementioned problems are solved and an advance in the art is achieved by providing an apparatus for detecting and comparing an amount of current sourced from within the telecommunication switch to the tip and ring drivers to a predetermined threshold current, and if the detected amount of current sourced from within the telecommunication switch to the tip and ring drivers is less than the predetermined threshold current, then a ground current indicating a pin fraud at the coin telephone has been determined.

In a specific embodiment, the aforementioned problems are solved by providing an apparatus for detecting an improper ground. This apparatus includes a ground connection terminal that is connected to more positive input of a ring voltage driver and also to a more positive input of a tip driver. A current detector senses an amount of current flowing from the first ground terminal to the tip and ring drivers. The ring driver and the tip driver each has a more negative input which is connected to a more negative voltage. The tip driver has an output terminal which is connected to a first lead of a first current detecting impedance. The second lead of the first current detecting impedance is connected to a first conductor. A coin telephone is also connected to the first conductor at a tip connection thereof at a distance from the first current detecting impedance. The ring driver also has an output terminal which is connected to a first lead of a second current detecting impedance. The second lead of the second current detecting impedance is connected to a second conductor. The coin telephone is also connected to the second conductor at a ring terminal thereof at a distance from the second current detecting impedance. The current detector detects an unauthorized connection between the tip conductor and ground if the amount of current flowing from the ground terminal drops below a pre-determined threshold.

According to another aspect of the invention, the aforementioned problems are solved by providing a method of detecting at a telecommunication switch a service fee collection disabling ground current at a telephone at a location that is remote to the telecommunication switch. The method includes the steps of determining a threshold loop current that is the lowest average amount of current that flows in a normal loop for an off-hook condition of the telephone, detecting an average amount of current that flows in a present loop for an off-hook condition of the telephone, and comparing the lowest average amount of current of the normal loop with the amount of current of the present loop for an off-hook condition of the telephone, and if the amount of current of the present loop is lower then terminating communication with the telephone.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a system for practicing aspects of the invention.

FIG. 2 is a logic flow diagram of a method for practicing the invention.

DETAILED DESCRIPTION

Referring to FIG. 1, an overall system 10 for making a coin telephone call is shown in block diagram form. Overall system 10 has a telecommunication switch 20 which is connected via tip and ring lines 30, 32 to a remote coin telephone 40. Lines 30, 32 may be simple wire conductors. The lines 30, 32 carry a composite signal that is made up of a loop current which flows when the hookswitch 42 is closed and the telecommunication switch 20 has not deactivated lines 30, 32 because of some past problem, such as an inadvertent left off-hook condition. This loop current is normally constant in magnitude, but for coin telephones, such as coin telephone 40, the direction of the current is briefly reversed by what is known in the art as a battery reversal. The purpose of these current reversals is to indicate when a fee for a call is due, as will be explained below. Superimposed on the loop current is an audio signal which typically carries the voices of the calling party and the called party. The operation of this audio signal is well known and will not be discussed further except to note that it is usually the reason each coin telephone call is made.

The coin telephone 40 is of the type that is used in Taiwan, R.O.C. and other nations. It is different from the coin telephones commonly used in the United States because it has no escrow bin where coins are held that may be returned if a call is not completed or if charges are reversed. Because of the lack of an escrow bin, the coin telephone 40 does not need the collect escrow coins signals or the refund escrow coins signals which are the major pin fraud problem in the United States. Further, the coin telephones in the United States originally used human operators to announce service fees, count the number and amounts of coins as well as collect the coins. Thus, historically a central operator position provided all the intelligence to charge and collect the fees for the service of making a call from a coin telephone. That central operator position was an early object of automation, which resulted in the United States having central, computer controlled operator position systems performing the coin telephone tasks that had previously been performed by human operators.

Sophisticated coin telephone 40 reflects a different approach that other telephone companies in other parts of the world have taken to coin telephone operation. Coin telephone 40 has a service fee meter and collection unit 44 instead of the coin escrow bin and collection bin common in the United States. The service fee meter and collection unit 44 is fairly autonomous in its charging and collecting of coins. Thus, this type of coin telephone uses more distributed automation than that commonly used in the United States. The telecommunication switch 20 supplies loop closure detection, dialed number collection, call setup and call tear-down for the coin telephone 40 in the way it provides those same services for non-coin telephones. The major difference is that the lines 30, 32 are identified as lines connected to a coin telephone, so the telecommunication switch 20 sends out current reversal pulses as the coin telephone call progresses to indicate to the service fee meter and collection unit 44 that another unit of service has been supplied by the telephone company and the coin telephone should collect a corresponding service fee in order to receive compensation for providing the coin telephone service and facilities. The current reversal used is a pulse of approximately 100 milliseconds and then the loop current is returned to the normal current flow direction. The current reversal has negligible effect on the voice circuits 46 or the handset 48.

Telecommunication switch 20 has switching circuits for telephone call set-up, routing, completion and tear-down. All of these are well known and, except as the routing determines the service fee schedule and service fee pulse initiating, are an incidental part of the present invention. Thus, they are not shown in FIG. 1.

Telecommunication switch 20 has audio and d.c. conditioning circuit 22 which respectively input the audio portion and the d.c. portion of the composite electrical signal to driver circuit 24. Driver circuit 24 amplifies the composite electrical signal and drives loop closure detector 27 with the amplified composite electrical signal. Audio and d.c. conditioning circuit 22 develops inputs to driver circuit 24 which provides for an audio portion of a call from another station (not shown) that is eventually presented to the earpiece of handset 48 and for a d.c. loop current which can be reversed in order to ultimately instruct service fee meter and collection unit 44 to collect a service fee. Driver 24 receives these inputs and uses tip driver 25 and ring driver 26, which may be integrated circuits, to amplify and drive the audio signal and the loop current around the loop formed by loop closure detector 27, lines 30, 32 and coin telephone 40.

Loop closure detector 27 has two impedances 28 and 29 which detect when hook switch 42 closes, e.g. by handset 48 being lifted off-hook. When hook switch 42 is open for many seconds the audio signals are disconnected by removal of any previous call and d.c. loop current cannot flow. For this open hook switch condition, normally the output of the tip driver 25 is near ground potential and the output of the ring driver 26 is near -48 volts. Upon going off-hook from the on-hook condition, loop current flows out of the tip driver 25, through impedance 28, line 30, coin telephone 40, line 32, impedance 29 into the output of ring driver 26. Loop closure circuit 27 senses the current in impedances 28 and 29 to determines from both of the sensed currents if hook switch 42 goes to an off-hook condition. Very little leakage occurs in lines 30, 32 and the coin telephone 40, so the magnitude of the current flowing through impedance 28 is essentially the same as the magnitude of the current flowing through impedance 29. A current imbalance detection between impedance 28 and impedance 29 is one way to practice the present invention and detect a fraud ground; however, the higher voltage levels, differential reference and the component ruggedness required because of environmental electrostatic discharges, e.g. lightning, means this would be a higher cost implementation than locating the current detector closer to ground terminal 21.

Since there is some difficulty in measuring loop currents at various locations around the loop in order to determine loop closure status, one embodiment of the present invention to detect pin fraud uses the level of current flowing from ground terminal 21 to tip driver 25 during normal current flow or to ring driver 26 during reverse current flow. To measure the amount of current flowing out of ground terminal 21, current detector 50 is used. Current detector 50 has a current sensor which may be a small valued impedance, such as a resistor, to turn the current into a voltage in a predictable way or it may be an induction coil or hall effect device that measures the magnetic field produced by the loop current. Any of these devices takes such a small amount of energy that it has a negligible effect on the overall operation of the loop circuit. In addition, current detector 50 has a detection circuit, which in the embodiment shown in FIG. 1 is a comparator (not shown). This comparator compares the voltage developed across the current sensor and compares it to a reference 52. If the voltage developed across the current sensor is less than the reference 52, then the comparator changes state and a pin fraud has been detected. Once a pin fraud has been detected, service is cut off. This service cut off may be by cutting off the loop current, by cutting off the audio, or by some other means.

Pin fraud is perpetrated by first establishing a coin telephone call in the usual manner and then inserting a fraud ground into the tip driver portion of the loop circuit. The typical location for a fraud ground is as the tip line 30 connects to the coin telephone 40. This is shown by a small valued resistor, FG1, which is connected to ground and to conductor 30. The connection to tip line 30 is represented by a dashed line to indicate that the fraud ground FG1 is added after the call is initially established. An alternate location for a fraud ground is at the tip side of the lines that connect the handset 48 to the voice circuit 46. This is shown by small valued resistor FG2 , which is connected to ground and to the tip side line to handset 48. The connection to the tip side line is represented by a dashed line to indicate that the fraud ground FG2 is added after the call is initially established.

After a call has been established from coin telephone 40, telecommunication switch 20 transmits service fee metering pulses. These pulses are typically 100 milliseconds long. The service metering pulses, as mentioned before, are pulses of reversed loop current. Normally, the tip driver 25 is driven such that its output is approximately ground, and the ring driver 26 is driven such that its output is approximately -48 volts. During each service fee metering pulse, the tip driver is driven such that its output is approximately -48 volts, and the ring driver 26 is driven such that its output is approximately ground. Service fee meter and collection unit 44 uses the reversal of the current or the reversal of the voltage between lines 30 and 32 as an indication that another amount of money should be charged and collected. If coins are appropriately inserted to pay for the call, the call is continued, otherwise service fee meter and collection unit 44 cuts off service to handset 48.

If either fraud ground FG1 or FG2 is inserted after a call is established, during normal operation there will be essentially no potential difference between ground terminal 21 and fraud ground FG1 or FG2, and substantially reduced current will flow through ground terminal 21, current detector 50, tip driver 25, loop closure impedance 28 and tip line 30. In such a case, fraud ground FG1 or FG2 operates as the source of the current flowing through service fee meter and collection unit 44, voice circuit 46, handset 48, ring line 32, loop current detector 29, ring driver 26 to -48 volt terminal. During reverse current operation, there will be essentially no potential difference between ground terminal 21 and fraud ground FG1 or FG2 and there will be substantially reduced current flowing through ground terminal 21, current detector 50, ring driver 26, loop closure impedance 29, ring conductor, service fee meter and collection unit 44, voice circuit 46 to fraud ground FG1 or FG2. Fraud ground FG1 or FG2 will act as a source of the current flowing from FG1 or FG2 through ring line 30, loop closure impedance 28, tip driver 25 to -48 volt terminal. During the current reversal there is negligible voltage and negligible current flowing through service fee meter and collection unit 44, which has the effect of disabling the operation of the current reversal metering pulses transmitted by telecommunication switch 20, thereby fraudulently limiting a fee for a coin telephone call to the fee that is collected before the fraud ground FG1 or FG2 is inserted.

It is worth noting that for a fraud ground situation, a great majority of the current flowing is sourced from the fraud ground FG1 or FG2 at the coin telephone location instead ground terminal 21 in the telecommunication switch 20. This is true of both normal loop current flow and reverse loop current flow. Thus, by carefully monitoring the current flowing from ground terminal 21 with current detector 50 and reference 52, a pin fraud is detected whenever a fraud ground FG1 or FG2 provides a bypass source of current. For such a case, the current through the current sensor of current detector 50 will have reduced output. The reduced output will be less than the minimum loop current represented by threshold reference 52 and the comparator within current detector 50 will change state and indicate the presence of a fraud ground, or an open loop condition (i.e. an on-hook signal). For either the fraud or the on-hook condition, it is appropriate to remove service and tear down the call until the next call is attempted.

FIG. 2 shows a method 200 according to the operation of the present invention to prevent the defrauding of coin telephones. Referring to both FIGS. 1 and 2, the method of preventing pin fraud will be explained. The method 200 includes the step 202 of pre-determining a threshold minimum loop current for a properly operating coin telephone call. The next step 204 tests to determine if loop closure current is flowing, i.e. an off-hook condition has occurred at the coin telephone 40. If the result is no loop closure current, step 204 re-cycles to test again for loop closure. If the result is loop closure current is flowing, the method 200 proceeds to step 206. Step 206 sets up a call with coin telephone 40. This set up includes the sourcing of the normal d.c. loop current from ground terminal 21 under the control of drivers 25 and 26. Once the call is set up, step 208 detects the loop current flowing from ground terminal 21 using the current sensor portion of the current detector 50. Next, step 210 compares the amount of loop current flowing from ground terminal 21 with the pre-determined threshold minimum loop current. If the amount of loop current is equal to or greater than the threshold minimum loop current, the method 200 re-cycles to step 208 to periodically test for pin fraud for the duration of the call. If the loop current is less than the threshold minimum loop current, the method 200 proceeds to step 210. At step 210 the comparison of the loop current flowing from ground terminal 21 has fallen below the pre-determined threshold minimum loop current and that indication from current detector 50 instructs switch 20 to terminate the present call with coin telephone 40. This termination can be by operation of electrical signals directly on the conditioning circuit 22 or the driver circuit 24, or by setting a software readable logic level which instructs the switch 20 to terminate and tear down the call as if there had been a normal hang-up of the handset 48. It is worth noting that method 200 will arrive at termination step 212 because a pin fraud at coin telephone was detected, or because the party at coin telephone 40 placed the handset 48 on-hook in a normal termination. Either case will cause the loop current flowing from ground terminal 21 to go to a very small value. Thus, the pin fraud detecting method 200 uses an existing call termination sequence and after the detection steps does not require any special treatment. Since step 212 termination treats the end of the call as a normal termination, method 200 returns to step 204 to test for the start of a next call. If a pin fraud is not removed after termination at step 212, a subsequent termination by operation of the steps 204-212 will quickly follow, thereby preventing and discouraging pin fraud.

Thus, it will now be understood that there has been disclosed a method and apparatus for reducing revenues losses from pin fraud. While the invention has been particularly illustrated and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form, details, and applications may be made therein. It is accordingly intended that the appended claims shall cover all such changes in form, details and applications which do not depart from the true spirit and scope of the invention. 

What is claimed is:
 1. An apparatus for use with a remote coin telephone for preventing coin telephone fraud comprising:a local telecommunications switch having a tip conductor and a ring conductor and a fraud detector local to said telecommunications switch for detecting an amount of current sourced from the telecommunications switch to a tip driver and comparing that amount of current with a predetermined threshold current, and if the detected amount of current sourced from within the telecommunication switch to the tip driver is less than the predetermined threshold current then a pin fraud ground current at the coin telephone has been determined.
 2. Apparatus according to claim 1, wherein said fraud detector includes a hall effect device to measure the current between the telecommunications switch and the tip driver.
 3. Apparatus according to claim 1, wherein said fraud detector includes a resistor through which said detected amount of current sourced from within the telecommunication switch to the tip driver flows, and said resistor develops a voltage proportional to said amount of current sourced from within the telecommunications switch to the tip driver.
 4. Apparatus for detecting an improper ground comprising:a ground connection terminal; a current detector having a first lead connected to said first ground terminal and a second lead; a ring voltage driver having a more positive input terminal connected to said second lead of said current detector, a less positive input terminal connected to a more negative voltage; and a ring driver output terminal; a tip voltage driver having a more positive input terminal connected to said second lead of said current detector, a less positive input terminal connected to said more negative voltage, and a tip driver output terminal; a first current loop detecting impedance having a first lead connected to said tip driver output terminal and a second lead; a tip conductor having a connection to said second lead of said first loop current detecting impedance; a second current loop detecting impedance having a first lead connected to said ring driver output terminal and a second lead; a ring conductor having a connection to said second lead of said second loop current detecting impedance; and a coin telephone having a first terminal connected to said tip conductor and a second terminal connected to said ring conductor; said current detector for detecting an unauthorized connection between said tip conductor and ground.
 5. Apparatus according to claim 4, wherein said unauthorized connection that is detected by said current detector is caused by equipment failure and service to said coin telephone is disabled.
 6. Apparatus according to claim 4, wherein said unauthorized connection that is detected by said current detector is a fraud connection for preventing said coin telephone from operationally receiving a fee-for-service charging pulse, and coin telephone service is disabled.
 7. Apparatus according to claim 4, wherein said ground connection terminal is located at a local telecommunication switch.
 8. Apparatus according to claim 4, wherein said more negative voltage is -48 volts.
 9. Apparatus for detecting an improper ground comprising:a ground connection terminal connected to ground; a current detector having a first lead connected to said first ground terminal and a second lead; a ring voltage driver having a more positive input terminal connected to said second lead of said current detector, a less positive input terminal connected to a more negative voltage; and a ring driver output terminal; a tip voltage driver having a more positive input terminal connected to said second lead of said current detecting means, a less positive input terminal connected to said more negative voltage, and a tip driver output terminal; a first current loop detecting resistor having a first lead connected to said tip driver output terminal and a second lead; a tip conductor having a connection to said second lead of said first loop current detecting impedance; a second current loop detecting impedance having a first lead connected to said ring driver output terminal and a second lead; a ring conductor having a connection to said second lead of said second loop current detecting impedance; a coin telephone having a first terminal connected to said tip conductor and a second terminal connected to said ring conductor; wherein if said coin telephone is in an off-hook condition during forward current flow current flows from said ground connection terminal through said tip voltage driver to said coin telephone to said ring voltage driver and said more negative voltage and during reverse current flow, flows from said ground connection terminal through said ring voltage driver to said coin telephone and to said tip voltage driver and said more negative voltage, said reverse current flow occurring in short pulses to cause a fee-for-service charge at said coin telephone; and wherein said current detector is for detecting a connection between said tip conductor and ground that would prevent said current reversals from causing said fee-for-service from being charged.
 10. Apparatus according to claim 9, wherein said unauthorized connection that is detected by said current detector is caused by equipment failure and service to said coin telephone is disabled.
 11. Apparatus according to claim 9, wherein said unauthorized connection that is detected by said current detector is a fraud connection for preventing said coin telephone from operationally receiving a fee-for-service charging pulse, and coin telephone service is disabled.
 12. Apparatus according to claim 9, wherein said ground connection terminal is located at a local telecommunication switch.
 13. Apparatus according to claim 9, wherein said more negative voltage is -48 volts.
 14. A method of detecting at a telecommunication switch a service fee collection disabling ground current at a remote telephone at a location that is remote to the telecommunication switch comprising the steps of:determining a threshold loop current that is the lowest average amount of current that flows in a normal loop for an off-hook condition of the remote telephone; detecting an average amount of current that flows in a present loop for an off-hook condition of the remote telephone; and comparing said threshold loop current that is the lowest average amount of current of said normal loop with said amount of current of said present loop for the off-hook condition of the remote telephone and if said amount of current of said present loop is lower than said threshold loop current that is the lowest average amount of current of said normal loop then terminating at the telecommunication switch communication with the remote telephone. 